Implement MouseEvent buttons attribute.

Source/core/events/MouseEvent.h

Index: Source/core/events/MouseEvent.h
diff --git a/Source/core/events/MouseEvent.h b/Source/core/events/MouseEvent.h
index 7d77582ba2a91605fb24c8f314415a2610d5229b..17a00609753a3d0ea24ce552ce1af1fa988c79e1 100644
--- a/Source/core/events/MouseEvent.h
+++ b/Source/core/events/MouseEvent.h
@@ -45,6 +45,7 @@ struct MouseEventInit : public UIEventInit {
     bool shiftKey;
     bool metaKey;
     unsigned short button;
+    unsigned short buttons;
     RefPtrWillBeMember<EventTarget> relatedTarget;
 };
 
@@ -59,7 +60,7 @@ public:
     static PassRefPtrWillBeRawPtr<MouseEvent> create(const AtomicString& type, bool canBubble, bool cancelable, PassRefPtrWillBeRawPtr<AbstractView>,
         int detail, int screenX, int screenY, int pageX, int pageY,
         int movementX, int movementY,
-        bool ctrlKey, bool altKey, bool shiftKey, bool metaKey, unsigned short button,
+        bool ctrlKey, bool altKey, bool shiftKey, bool metaKey, unsigned short button, unsigned short buttons,
         PassRefPtrWillBeRawPtr<EventTarget> relatedTarget, PassRefPtrWillBeRawPtr<DataTransfer>,
         bool isSimulated = false, PlatformMouseEvent::SyntheticEventType = PlatformMouseEvent::RealOrIndistinguishable);
 
@@ -77,6 +78,7 @@ public:
     // WinIE uses 1,4,2 for left/middle/right but not for click (just for mousedown/up, maybe others),
     // but we will match the standard DOM.
     unsigned short button() const { return m_button; }
+    unsigned short buttons() const { return m_buttons; }
     bool buttonDown() const { return m_buttonDown; }
     EventTarget* relatedTarget() const { return m_relatedTarget.get(); }
     void setRelatedTarget(PassRefPtrWillBeRawPtr<EventTarget> relatedTarget) { m_relatedTarget = relatedTarget; }
@@ -100,7 +102,7 @@ protected:
     MouseEvent(const AtomicString& type, bool canBubble, bool cancelable, PassRefPtrWillBeRawPtr<AbstractView>,
         int detail, int screenX, int screenY, int pageX, int pageY,
         int movementX, int movementY,
-        bool ctrlKey, bool altKey, bool shiftKey, bool metaKey, unsigned short button,
+        bool ctrlKey, bool altKey, bool shiftKey, bool metaKey, unsigned short button, unsigned short buttons,
         PassRefPtrWillBeRawPtr<EventTarget> relatedTarget, PassRefPtrWillBeRawPtr<DataTransfer>,
         bool isSimulated, PlatformMouseEvent::SyntheticEventType);
 
@@ -110,6 +112,7 @@ protected:
 
 private:
     unsigned short m_button;
+    unsigned short m_buttons;
     bool m_buttonDown;
     RefPtrWillBeMember<EventTarget> m_relatedTarget;
     RefPtrWillBeMember<DataTransfer> m_dataTransfer;